The present invention relates to a process for analysing a sample by emission spectrography in which a laser beam is directed onto the sample with a sufficient intensity to volatilize, ionize and excite the bodies constituting the said sample. The volatilized material constitutes a plasma. Only a very small part of the sample is volatilized at the impact point of the laser beam. The plasma created by the volatilized and excited body is observed by means of a suitably positioned spectrograph.
The invention also relates to a sample analysis apparatus using a laser, a spectrograph and a target sample located in a suitably positioned enclosure.
Analysis by emission spectrography comprises exciting the atoms or molecules of a sample to be analysed in such a way that they undergo transitions. When these atoms or molecules redescend to their fundamental state, they emit light at wave lengths which are characteristic of the constituents present in the sample. It has been a preferred method for analysing substances which have been widely used in the analysis of steels. In general, in emission spectrography applied to solid bodies such as metals, one of the bodies to be studied constitutes an electrode and an electrical current is passed between two electrodes which volatilizes and excites the atoms of the constituent or body which it is desired to analyse.
However, emission spectrography by arc (or by spark, which is a very similar process and well known to the skilled expert) has certain disadvantages and certain limitations of use. The body to be analysed must be a good conductor of electricity in order to conduct the incoming current to the electrode. Moreover, it must be machined so that it has a suitable geometry for forming an electrode. However, in the case of irradiated elements which it is desired to dose, these operations are difficult and expensive.
Thus, the arc spectrum method cannot be applied to very poor conductors such as glass, used in particular for the vitrification of nuclear fission products.
Moreover, in the case of irradiated fuels, the handling operations and machining made necessary by the production of the electrodes are a source of radio active pollution which it is indispensible to avoid in most cases. Finally, the inspection of materials by spark or arc spectrum is in part destructive.
Thus, compared with the use of conventional electrical sources for the volatilization or excitation of materials, for emission spectrography, the use of a laser beam has definite advantages, more particularly in the analysis of glass. The process and apparatus according to the invention use a laser beam focused on a sample for volatilizing and exciting the bodies forming said sample. The observation of the light spectrum emitted by the volatilized and excited bodies makes it possible to determine the constitution of the sample.
Preferably, a power laser is used for both volatilizing and exciting the atoms (or optionally molecules) of the sample. The power laser makes it possible to dispense with auxiliary power sources for exciting the gases obtained by laser sputtering used in the prior art combined with a low power laser.
It has been found that both laser excitation and volatilization are substantially independent of the precise physical-chemical characteristics of the sample material, permitting in the case of sufficiently powerful lasers to dose any random body, said dosing being both qualitative and quantitative and applicable to non-conductive refractory materials and conductive metallic materials.
Moreover, the laser only volatilizes a small proportion of the material, permitting a non-destructive analysis to be performed. Compared with the arc spectrum, the use of a spectrum obtained by a laser is polyvalent, the use of a power laser permits the elimination of auxiliary excitation means and requires no special preparation of the sample which decreases the analysis time, the pollution risks and contamination.
No preparation of the surface of the sample is necessary and said sample can have a random shape and volume. Finally, the focusing of the laser beam, which is very precise (approximately the wave length of the laser used) makes it possible to envisage an analytical cartography of the sample, by varying the position of the impact point.
In a known powder analysis method using a power laser beam, the spectrograph used is positioned with its axis parallel to the surface of the sample at the impact point of the laser beam. This leads to a short measuring period due to the speed of expansion of the plasma produced perpendicular to the surface of the sample and a measurement of the concentration of the sample in various bodies which depends greatly on the plasma zone observed by the spectrograph. Moreover, in this type of arrangement, the spectrograph slit is not well illuminated. Finally, as the laser beam reaches the surface perpendicularly, a large proportion of the light is reflected in accordance with the principle of inverse return of light, which may damage the actual laser.
Finally, emission spectrography has not hitherto been performed under a controlled atmosphere, more specifically a vacuum which, as will be shown hereinafter, permits a considerable improvement of the structures of the emission lines. The decrease in the pressure of the gas in which the sample is located makes it possible to reduce the auto-absorption effect. Thus, when radiation is emitted by a gas due to the transition from a high level to a low level of the atom or a sufficiently populated level, this radiation can again be absorbed by the gas. As emission takes place in a region which is hotter than the external gas surrounding the plasma core the emission line is wider at the hot points. This line is absorbed in the colder points corresponding to the narrow absorption rays. Thus, the emission line or ray has an absorption hollow in its centre which makes identification difficult. The use of a partial vacuum around the sample permits the avoidance of this phenomenon.